A construction of a turbine rotor, which is a base for the present invention, will now be described with reference to FIG. 1.
FIG. 1 is a partial front elevation of a radial turbine rotor comprising a turbine rotor shaft 7 and a turbine wheel 5. The turbine wheel 5 has a hub 50 having a truncated cone-like shape along the wheel rotational center line C-C, and a plurality of impellers 40 in the circumferential direction at approximately regular intervals around the hub 50. Between adjacent impellers 40, a web-like scallop 30 is formed by making a cutout. The scallop 30 is formed between a negative pressure side of an impeller 40 and a pressure side of an impeller 40 adjacent to aforementioned impeller 40. The minimum radius portion between the wheel rotational center line C-C and the inner edge of the scallop 30 is in an approximately intermediate position between the two impellers 40, 40. Accordingly, the scallop 30 has a symmetric shape with respect to the minimum radius portion. The scallop 30 has a role of reducing centrifugal stress and moment of inertia in the turbine wheel 5.
The rotor shaft 7 extends along the wheel rotational center line C-C on the back side of the turbine wheel 5 and is fixed on the turbine wheel 5. The rotor shaft 7 has an intermediate shaft portion 20 having a diameter larger than the rotor shaft mounted integrally thereon on an end side. The rotor wheel is fixed on the rotor shaft 7 via the intermediate shaft portion 20 (see Patent Document 2 and Patent Document 3).
The turbine wheel 5 is manufactured by casting. Accordingly, the casting itself is likely to have a biased weight, i.e. unbalance, with regard to the rotational center. If a turbocharger has a turbine rotor 1 with such a turbine wheel 5 having an unbalance, a centrifugal force is caused due to the unbalance when the turbine rotor rotates at high speed, which may leads to vibration of the turbocharger itself.
In view of this, in order to correct the balance of the turbine wheel manufactured by casting, conventionally an arc-like balance cut portion of which center is on the rotational center line C-C is formed on the back side of the turbine wheel.
In particular, a turbocharger for automobiles among such turbochargers, has been downsized for the purpose of improvement of fuel consumption, and the temperature of the exhaust gas tends to become higher due to increase in performance.
In response to such requirement for increasing performance, a turbine rotor having a turbine wheel composed of TiAl-based alloy which is excellent in heat resistance, which is joined to a shaft composed of steel with a brazing material such as Ni brazing material, is suggested. Such a turbine rotor is disclosed by Non-Patent Document 1, for example.
As the above turbine wheel 5 used for a turbocharger for automobiles is manufactured by casting even though precision casting, it cannot be processed while the rotational balance is kept with regard to the rotational center (c) in the circumferential direction, as in the case of machine processing. Accordingly, conventionally, on the back side of the hub of the turbine wheel 5 obtained by precision casting, a balance cut portion 11 is formed by cutting in an arc-like shape along the circumferential direction of the wheel by means of cutting instrument such as an end mill, or, a boss portion 12 on an end side of the hub 50 is cut, to correct the rotational balance.
The arc-like balance cut portion 11 on the back side of the wheel is preferably formed near the edge of the scallop which is on the outer position than the intermediate shaft portion 20 which is on the rotational center side with a view to correcting the rotational balance. However, since TiAl forming the wheel is a brittle material, if the balance cut 11 is intended to be formed near the edge side of the scallop, the press force in cutting by cutting instrument such as an end mill my propagate to the scallop portion 30 of the impeller, whereby cracks or fracturing is likely to be caused in the scallop portion 30. Further, if the wheel having cracks or fracturing in the scallop portion 30 is rotated at high speed, the cracks or fracturing may be enlarged in the wheel of a brittle material, and the turbine wheel 5 may be damaged during operation.
The reason why cracks is caused in the scallop portion 30 is such that as shown in FIG. 1, since a rotating cutting instrument such as an end mill is pressed on the back side of the turbine wheel 5 to perform cutting processing, a press force acts on the scallop portion 30, whereby cracks is caused in brittle TiAl. On the other hand, Patent Document 1 discloses a technique of correcting the rotational balance by using laser instead of cutting instruments.
However, in such technique, the turbine wheel 5 itself is not processed, but an impeller nut to fasten the impeller 40 is cut for self-aligning. Accordingly, such technique may be applied only to correcting rotational balance of a compressor wheel where the rotor shaft and the impeller are separated.
Further, in such know technique, “in a state where impeller nut is rotated so that the frequency of the vibration of impeller exceeds the primary resonance point where the amplitude becomes maximum, the impeller nut is cut from the front direction by a laser LS of which irradiation position is fixed for self-aligning”. Accordingly, such technique has a problem such that balance correction becomes cumbersome, and in particular, the position of the balance correcting part cannot be determined unless the impeller nut is rotated. Thus the technique is not suitable for mass production.
Further, in the technique, the impeller nut is cut by laser from the front side to correct the rotational balance, which is basically different from the present invention where a balance cut portion 11 is formed on the back side of the turbine wheel 5.